The present invention relates to a radiodirection finding method and apparatus using Doppler effect frequency modulation.
Different systems of aid to air or sea navigation are already known using Doppler effect frequency modulation. Among the known systems using such modulation may be mentioned the Doppler VOR system operating for transmission and the Rohde and Schwarz direction finder at reception. These systems are based on the switching of a high number of antenna strands (39 antenna strands for the Doppler VOR system and 16 antenna strands for the Rohde and Schwarz system) which are placed about a circumference, at equal distances from each other. These antenna strands are energized in a rotating sequence, in other words one after the other about the circumference, each of the strands being energized for a predetermind lapse of time, which corresponds to the rotation of a fictitious antenna about the center of the circumference. The bearing of a source transmitting a high frequency signal is obtained by measuring the phase shift between an antenna rotation reference signal and the signal obtained from the fictitious rotating antenna, after demodulation by a frequency modulation receiver.
The accuracy of such a direction finding system depends on the number of antenna strands, on the form of the switching signals, on the reproducibility of the circuits associated with eacch antenna strand (switching, amplification, dimensional characteristics) and on the phase stability of the receiver or on a fixed phase shift which may be compensated for.
To overcome the difficulties related to the accuracy required of the devices switching the antenna strands and their controls, the simplest solution consists in increasing the number of antenna strands. In fact, the accuracy required for the shape of control signals and the switching devices is all the more reliable the higher the number of antenna strands. However, this results in a bulky apparatus, and yet does not solve the problem of the phase shift of the receiver.